So how should you decide what size
wastegate to run on your turbocharged vehicle? Well, let’s first look at how a wastegate works. For purposes of this discussion we will reference two very popular wastegate sizes, 38mm and 44mm.
The purpose of a wastegate is to vent exhaust gas pressure away from the exhaust wheel of the
turbo in order to control shaft speed. The more exhaust gas the wastegate vents, the slower the turbine wheel will spin. For a turbocharged car that sees primarily street use, most people think the smaller 38mm wastegate would work best, and although it is not a bad choice, ultimately the 44mm would work better for most street applications. A street car will not be running the turbocharger at its peak efficiency all the time.
Because of this, boost will be kept on the lower end of the turbo’s efficiency range. Since the boost will remain generally low, a larger volume of exhaust gas will need to be vented prior to the turbine exhaust wheel. That is why the 44mm wastegate would be a better choice for this application than the 38mm. It can more efficiently vent excess pressure from the system with the same valve lift as the 38mm.
If you were sizing a wastegate for a race application, a smaller wastegate would generally be a more efficient choice. In race applications, boost pressures are higher, so a turbo is more likely to be operating in its peak efficiency range for longer periods of time. That means you would want to keep more exhaust pressure going through the turbo, which means you are venting less exhaust gas. In this instance the 38mm wastegate, since it is smaller, will vent less gas and keep more pressure in the system to spin the turbo.
